1. Field of the Invention
This invention relates generally to the field of boring and penetrating the earth, and more particularly to methods and apparatus for boring by fluid erosion.
2. Description of the Prior Art
High pressure water jets have been used in the continuous mode for drilling through rock or other hard material, as exemplified by the patent to Summers U.S. Pat. No. 4,119,160. High pressure water jets have also been used in the long wall mining of coal by using continuous jets mounted so as to oscillate in a plane. A device of this general type is exemplified by the application to Barker et al Ser. No. 894,769 filed Apr. 10, 1978 now U.S. Pat. No. 4,265,487 issued May 5, 1981.
Intermittent high pressure water jets for drilling are also known and exemplified by the patent to Hall et al U.S. Pat. No. 3,927,723 entitled "Apparatus for Drilling Holes Utilizing Pulsed Jets of Liquid Charge Material" and also the patent to Cooley U.S. Pat. No. 3,520,477 entitled "Pneumatically Powered Water Cannon".
Each of the devices described in the two above patents is capable of producing intermittent or pulsed water jets of very high velocity. The cutting capability of the pulsed jets are determined solely by the energy imparted by an activating piston. Pressure pulses formed in this manner are reflected back into the system and have a destructive effect on the equipment.
Scientific studies of shaped explosive charges have shown that fast jets of metallic liquids are formed in the collapse of such charges. The velocity achieved is a function of the charge size and shape and material composition of the liner which upon collapse, will create the cutting jet. Examples of such studies are:
1. Koski, W. S., F. A. Lucy, R. G. Shreffler, and F. J. Willig, "Fast Jets from Collapsing Cylinders," Journal of Applied Physics, Vol. 23, No. 12, December 1952. PA1 2. Walsh, J. M., R. G. Shreffler, and F. J. Willig, "Limiting Conditions for Jet Formation in High Velocity Collisions," Journal of Applied Physics, Vol. 24, No. 3, March 1953. PA1 3. Dunne, B. and B. Cassen, "Some Phenomena Associated with Supersonic Liquid Jets," Journal of Applied Physics, Vol. 25, No. 5, May 1954. PA1 4. Crane, L., S. Birch, and P. D. McCormack, "The Effect of Mechanical Vibration on the Break-up of a Cylindrical Water Jet in Air," Journal of Applied Physics, Vol. 15, 1964. PA1 5. Dabora, E. K., "Production of Monodisperse Sprays," The Review of Scientific Instruments, Vol. 38, No. 4, April 1967.
The jets so produced and described in these studies are transient and are not produced on a continuous basis. This limits their possible fields of application, e.g. to the perforation of well casings, and the like.
Scientific studies have also been made of fluid streams and their break-up into droplets. These latter studies are exemplified by:
The external augmentation of the velocity of fluid jets has also been studied and reported by the Bowles Engineering Corporation in 1967 in a report to the Department of Transportation (R-12-21-67, Contract No. 7-35380). This study had to do with the impacting of two relatively slow-moving slugs of water to produce a very small but effective fast jet. The work described combined individual fluid slugs and was done on a cyclical rather than continuous basis.
The phenomenon of "accumulative effect" has been employed in a continuous system as reported by Mazurkiewicz et al.: Adaptation of Jet Accumulation Techniques for Enhanced Rock Cutting, ASTM Conference on Erosion: Prevention and Useful Applications, Vail, Colo., Oct. 24-26, 1977.
It is also known that the break-up into droplets of a stream of flowing liquid can be accelerated if it is excited at a particular frequency. The frequency at which a fluid stream is most unstable is commonly referred to as the Rayleigh instability frequency and was first reported by J. W. S. Rayleigh proceedings of London Mathematical Society, Volume 10, No. 4, 1878.